1. Field of the Invention
The present invention relates to an electrophotographic image forming apparatus, particularly to an image forming apparatus which forms a toner image on a sheet of paper such as embossed paper having an uneven surface.
2. Description of the Related Art
An image forming apparatus such as an electrophotographic printer or copier forms a toner image on an image carrier such as a photoreceptor, transfers the formed toner image onto a sheet of paper, and then performs heating/pressure fixing to obtain the sheet of paper on which the toner image is formed.
As the range of uses of the copier and printer has been increasing in recent years, paper with various paper qualities including embossed paper, the surface of which is embossed, has come into use in addition to plain paper having a smooth surface.
A problem with the paper having a highly uneven surface such as the embossed paper is that a toner is not sufficiently transferred to a concave portion of the paper, resulting in decreased uniformity of an image.
An image forming apparatus disclosed in JP 2006-267486 A and JP 2008-185890 A realizes improved uniformity in a post-transfer image by imparting an electrical charge that has an opposite polarity to that of the toner to a transfer surface of the embossed paper with use of a pre-transfer charge roll.
JP 2008-262085 A discloses an image forming apparatus which imparts in advance an electrical charge having the same polarity as that of the toner to a convex portion of the embossed paper with use of a pre-transfer electrical charge imparting unit such as an electrode. As a result, a transfer electric field relatively stronger in the concave portion than in the convex portion can be created in a secondary transfer nip, so that the uniformity in the post-transfer image is improved across the paper surface by setting an optimized secondary transfer bias to the concave portion.
With the technology disclosed in JP 2006-267486 A and JP 2008-185890 A, however, the toner is not well transferred to the concave portion of the embossed paper when the amount of electrical charge imparted to the embossed paper by the charge roll is insufficient, in which case the image has imperfect uniformity. When the excessive amount of electrical charge is imparted to the embossed paper, on the other hand, one can expect the toner to be better transferred to the concave portion whereas in the convex portion, the excessive amount of electrical charge causes local electrical discharge to the toner and thus causes new defective transfer such as so-called transfer cissing and scattering.
Moreover, the technology disclosed in JP 2008-262085 A imparts an electrical charge that cancels the transfer electric field to the convex portion of the paper, thereby making it less prone to result in the defective transfer accompanying the electrical discharge to the toner in the convex portion. This technology improves transfer performance to the concave portion by increasing the secondary transfer bias. However, there is a limitation to how much the transfer performance is improved by increasing the secondary transfer bias in the case of the paper having the highly uneven surface such as the embossed paper because the defective transfer caused by the electrical discharge to the toner occurs before the transfer performance to the concave portion is sufficiently improved, where the transfer performance to the concave portion and the defective transfer caused by the electrical discharge cannot be improved at the same time.